1. Field of the Invention
The present invention relates to an apparatus for measuring the quantity of a liquid such as a fuel in a tank and more particularly to such apparatus of the type for capacitively detecting a liquid level in the tank to indicate the liquid quantity.
2. Description of the Prior Art
A capacitor type liquid quantity measuring apparatus per se has been disclosed, for instance by U.S. Pat. No. 3,208,280 issued on Sept. 28, 1965. Such an apparatus utilizes principle that, if there is a dielectric substance such as air, fuel, water or the like between a pair of opposing positive and negative electrode plates, the electrode plates form a capacitor together with a medium of the substance; and thus when the electrode plates are arranged vertically in a vessel such as a tank, a surface area of the electrode plates dipped in the fuel is varied substantially depending on fuel level in the tank so as to show a change of capacitance, which corresponds to the fuel level. In this case, it is of course necessary to arrange another pair of electrode plates always dipped fully in the fuel, which measures a dielectric constant variation due to the temperature of the fuel to compensate the relationship between the capacity measured by the first pair of the electrode plates and the level of the fuel in the tank. In said U.S. patent, the capacitive information from said both pairs of electrode plates are fed through an amplifier to a motor which operates a wiper of potentiometer and an indicator to convert the capacitive informations into the information of fuel level and to indicate the same on the indicator. However, such known system is not suitable for measuring the quantity of fuel in each of plural tanks to centrally conduct the fuel, since as aforesaid, the capacity is quite sensitive to the change of temperature and thus if the capacitive information is fed in this form over a relatively long distance, the value changes to such an extent that any correction is impossible.
The above U.S. patent is directed mainly to the fuel gauging apparatus for airplanes and gives neither disclosure nor suggestion for detecting undesired water which possibly accumulates at bottom of the tank. This detection is very important matter in order to remove water from fuel accommodated in underground tanks, such as in fuel dispensing stations. Namely, such a tank has a ventilation opening communicating with an inside of the tank from a safety view-point and thus rainwater may enter in the tank through the opening to form a water layer under the fuel layer and if a flood is caused, a large amount of water may also enter in the tank through the opening. Even if such exceptional occasions are excluded, further, the fuel such as gasoline for vehicles contains inevitably some quantity of water and thus the accumulation of water can not be avoided due to a separation thereof from the fuel during the storing of the fuel. Further, the temperature difference between the inside of tank and atmosphere causes a condensation of moisture in air as water drops at inner surface of a pipe communicating with said ventilation opening, said water drops flowing down through the surface into the fuel tank to increase water level therein.
Even if the water layer has already been formed in the fuel tank due to at least one of said or other causes, each of known methods for measuring the quantity of fuel based on a level of fuel in the tank indicates the remaining fuel quantity under an assumption that water is not present in the tank. It is, therefore, necessary to set the lower level for dispensing the fuel at a relatively high point, in order to avoid a possible supply of water-containing fuel to vehicles and the like. This means that the dispensable fuel amount in the tank decreases and that a careful operation is required on removal of the water by suction, since the amount of water accumulated in the tank can not be confirmed.